1. Field of the Invention
This disclosure relates generally to a valve to relieve a gaseous pressure in a branching unit having at least three limbs, one of them being for an inspired gas, a second one being for an expired gas and a third one being for both the inspired and expired gases and being in flow communication with lungs of a subject. Also this disclosure relates generally to a method to relieve gaseous pressure in a branching unit providing a breathing gas for a subject inhalation and receiving a breathing gas exhaled. Further this disclosure relates generally to an arrangement for ventilating lungs of the subject.
2. Description of Related Art
During ventilation, a patient's lungs are connected with an artificial ventilation system with breathing circuit. For inspiration during artificial ventilation, the patient's lungs are filled using a ventilator utilizing overpressure. An overpressure pneumatic ventilator adds drive gas to the breathing circuit which forces the breathing gas to the patient's lungs. Alternatively, piston driven ventilators reduce the breathing circuit volume while forcing the gas to flow to the lungs. Delivering additional fresh breathing gas to the lungs using a gas mixer may also increase the breathing circuit and lung pressure and thus participate in the inspiration.
A clinician may also use a manual breathing bag, a flexible container connected pneumatically on the breathing circuit, to pressurize the circuit and patient's lungs for an inspiration. Squeezing the manual bag reduces the breathing circuit volume which increases the circuit pressure and forces the gas to flow to the patient's lungs.
During expiration the overpressure is released by opening a valve allowing the gas to flow out from the pressurized lungs. For the purpose of relieving pressure, the ventilator uses an expiration valve. A control algorithm regulates the flow through the valve in order to reach and maintain required expiration pressure.
During manual ventilation, releasing the bag increases the circuit volume allowing the gas to flow out from the patient's lungs, which results in a reduction of pressure in the lungs. Additional pressure developed in the breathing circuit is released through an adjustable pressure limiting (APL) valve, with which the clinician limits the maximum breathing circuit pressure. Any pressure exceeding the APL limit pressure is relieved through the valve.
An overpressure is the most serious safety risk related to patient ventilation. As a response to a sudden rise in pressure, lung damage may occur before manual relief is possible. Therefore, the ventilation system must be equipped with safety measures configured to automatically prevent the overpressure in normal operation, as well as overpressure resulting from a on a device in, what would otherwise be, single failure operation condition.
In addition to the overpressure, an inability to relieve the breathing circuit pressure may cause a static elevated, sustained, pressure. This compromises gas exchange in the lungs and may cause cardiologic complications.
Situations that may cause an unintentional breathing circuit pressure rise include ventilator or gas mixer failures that limit the gas delivery to the breathing circuit, pressure increases caused by external reasons like patient coughing, or breathing circuit occlusion which prevents or slows down the pressure relief from the breathing circuit.
Particularly in situations involving occlusion of the expiration pathway of the patient, no exhalation gas can be evacuated through the ventilator expiration valve or the APL valve because the gas pathway is blocked. For such situation ventilator safety regulations require a protection against a patient hazard arising in the normal operation or arising from any equipment single failure condition. The pressure at the patient connection port shall be limited to maximum 12.5 kPa. Ventilation systems are equipped with spring-loaded mechanical pressure relief valves for this. A relief limit below the standard requirement would however be desirable specifically for small patients. On the other hand ventilation of obese and some lung-sick patients may require the standard relief limit. Furthermore, during failure situations, these state-of-the-art protection devices only limit the pressure rise whereas total pressure relief would be desirable to stop hazardous sustained pressure. Such total relief can, however, not be continuous, because the ventilation system should still allow a continuation of manual ventilation in the case of ventilator failure or a power supply failure. To allow this, the safety pressure relief valve must remain closed.